The present invention relates to a cationically electrodepositable coating composition, more specifically to a cationically electrodepositable coating composition which comprises a specific blocked polyisocyanate cross-linking agent and has a low heating loss, a low gum soot property and a low temperature-curing property and which can form an electrodeposited coating film having a good coated surface smoothness.
Known as a cationically electrodepositable coating material is a thermosetting type cationically electrodepositable coating material comprising an amine-added epoxy resin and a blocked polyisocyanate as a cross-linking agent, and those obtained by blocking aromatic or aliphatic polyisocyanates with ether alcohol base blocking agents such as ethylene glycol butyl ether have so far been used as the blocked polyisocyanate from the viewpoints of a corrosion resistance of the coating film formed and a stability of the coating material. However, a cationically electrodepositable coating material using a polyisocyanate blocked with an ether alcohol base blocking agent as a cross-linking agent has the problems that a lot of gum and soot due to a heating loss content (a weight loss proportion of a coating film in curing by baking) are produced in a line oven and that the baking temperature is high.
On the other hand, a cationically electrodepositable coating material using a polyisocyanate blocked with a blocking agent of an oxime base such as methyl ethyl ketoxime and hexanone ketoxime as a cross-linking agent provides a coating film which can be cured at a relatively low temperature but has the problem that an aging stability of the coating material and a corrosion resistance of the coating film are inferior.
An object of the present invention is to provide a cationically electrodepositable coating material which has a low heating loss property and a low temperature-curing property and which is excellent in a stability as a coating material and a corrosion resistance of the coating film.
The present inventors have found that the object described above can be achieved by using a blocked isocyanate as an external or internal cross-linking agent component, which is formed by using some kind of a specific amide compound as a blocking agent, and they have come to complete the present invention.
Thus, the present invention provides a cationically electrodepositable coating composition (hereinafter referred to as the coating composition A of the present invention) comprising as a cross-linking agent, a blocked polyisocyanate having at least 0.1 blocked isocyanate group represented by Formula (I): 
wherein R1 represents a hydrogen atom, methyl, ethyl or propyl, and R2 represents methyl or ethyl.
Further, the present invention provides a cationically electrodepositable coating composition (hereinafter referred to as the coating composition B of the present invention) comprising as a base resin, an active hydrogen-containing cationic resin having at least 0.1 blocked isocyanate group per molecule on the average represented by Formula (I) described above.
The coating compositions of the present invention shall be explained below in further details.
The coating composition A of the present invention is a cationically electrodepositable coating composition comprising a cationic resin having an active hydrogen group capable of reacting an isocyanate group as a base resin and the blocked polyisocyanate having the blocked isocyanate group represented by Formula (I) described above as an external cross-linking agent.
Capable of being used as the active hydrogen-containing cationic resin used as the base resin in the coating composition A of the present invention are, for example, conventional cationically electrodepositable coating resins having an active hydrogen group such as a primary or secondary amino group and a hydroxyl group and a cationic group such as a primary amino group, a secondary amino group, a tertiary amino group and a quaternary ammonium group (when this cationic group contains active hydrogen, it can double as the active hydrogen group), which is required for making the resin water-soluble or water-dispersible, for example, resins of an epoxy base, an acryl base, a polybutadiene base, an alkid base and a polyester base. Among them, amine-added epoxy resins are particularly suitable.
The above amine-added epoxy resin includes, for example, (1) adducts of polyepoxide compounds to primary mono- and polyamines, secondary mono- and polyamines or primary and secondary mixed-polyamines (refer to, for example, U.S. Pat. No. 3,984,299); (2) adducts of polyepoxide compounds to secondary mono- and polyamines having a primary amino group which is reduced to ketimine (refer to, for example, U.S. Pat. No. 4,017,438); and (3) reaction products obtained by etherification reaction of polyepoxide compounds with hydroxyl compounds having a primary amino group which is reduced to ketimine (refer to, for example, Japanese Patent Application Laid-Open No. 43013/1984).
The polyepoxide compound used for producing the amine-added epoxy resin described above is a compound having at least two epoxy groups in a molecule and is suitably a compound having a number average molecular weight falling in a range of usually at least 200, preferably 400 to 4000 and more preferably 800 to 2000. In particular, a compound obtained by reacting a polyphenol compound with epichlorohydrin is preferred. The polyphenol compounds which can be used for producing the above polyepoxide compounds include, for example, bis(4-hydroxyphenyl)-2,2-propane, 4,4-dihydroxy-benzophenone, bis(4-hydroxyphenyl)-1,1-ethane, bis(4-hydroxyphenyl)-1,1-isobutane, bis(4-hydroxy-tert-butyl-phenyl)-2,2-propane, bis(2-hydroxynaphthyl)methane, tetra(4-hydroxyphenyl)-1,1,2,2-ethane, 4,4-dihydroxy-diphenylsulfone, phenol novolak and cresol novolak.
The above polyepoxide compounds may be those reacted partly with polyols, polyetherpolyols, polyesterpolyols, polyamideamines, polycarboxylic acids and polyisocyanate compounds. Further, they may be those graft-polymerized with xcex5-caprolactone and acryl monomers.
The blocked polyisocyanate used as a cross-linking agent in the coating composition A of the present invention can be produced by blocking at least one isocyanate group of a polyisocyanate compound having at least two isocyanate groups (NCO) in a molecule with an amide compound represented by the following Formula (II):
R1xe2x80x94COxe2x80x94NHxe2x80x94R2xe2x80x83xe2x80x83(II)
wherein R1 and R2 are synonymous with those defined above.
The polyisocyanate described above includes, for example, aromatic, aliphatic or alicyclic polyisocyanate compounds such as tolylenediisocyanate, xylilenediisocyanate, phenylenediisocyanate, bis-(isocyanatemethyl)cyclohexane, tetramethylenediisocyanate, hexamethylenediisocyanate, methylenediisocyanate and isophoronediisocyanate, and terminal isocyanate group-containing compounds obtained by reacting excess amounts of these polyisocyanate compounds with low molecular active hydrogen-containing compounds such as ethylene glycol, propylene glycol, trimethylolpropane, hexanetriol and castor oil. Among these polyisocyanate compounds, suited are aromatic diisocyanates, particularly diphenylmethane-2,4xe2x80x2-diisocyanate, diphenylmethane-4,4xe2x80x2-diisocyanate (usually called MDI) and a mixture (usually called crude MDI) of diphenylmethane-4,4xe2x80x2-diisocyanate with diphenylmethane-2,4xe2x80x2-diisocyanate and polymethylenepolyphenylisocyanate.
The amide compound represented by Formula (II) described above which is used for blocking these polyisocyanate compounds includes, for example, N-methylacetamide, N-ethylacetamide, N-methylpropionamide and N-methylformamide, and they can be used alone or in combination of two or more kinds thereof. Substantially all isocyanate groups of the polyisocyanate compound are preferably blocked with the above amide compound in order to reduce a heating loss, but a part of the isocyanate groups may be blocked, if necessary, with other conventional blocking agents which have so far been used. The blocking agents which can be used in combination include, for example, lactam base compounds such as xcex5-caprolactam and xcex3-butyrolactam; oxime base compounds such as methyl ethyl ketoxime and cyclohexanoneoxime; phenol base compounds such as phenol, p-t-butylphenol and cresol; aliphatic alcohols such as n-butanol and 2-ethylhexanol; aromatic alkylalcohols such as phenyl-carbinol and methylphenylcarbitol; and ether alcohol base compounds such as ethylene glycol monobutyl ether. Among them, the compounds having a relatively low molecular weight of 140 or less are suited.
A use amount of these blocking agents to the polyisocyanate compound falls suitably in a range of 1 to 1.3 mole per equivalent of the polyisocyanate group in total.
The polyisocyanate compound described above can be reacted with the blocking agent by a conventionally known method. The resulting blocked polyisocyanate is stable at a room temperature, but when it is heated at a baking temperature of the coating film, a temperature of usually about 100 to about 200xc2x0 C., preferably about 120 to about 160xc2x0 C., it dissociates the blocking agent to reproduce a free isocyanate, and this is reacted with an active hydrogen group of the base resin contained in the coating composition to cure the resin.
The coating composition A of the present invention can be prepared by neutralizing the active hydrogen group-containing cationic resin described above with a neutralizing agent such as aliphatic carboxylic acid to make the above resin water-soluble or water dispersible and then mixing it with the blocked polyisocyanate compound described above. The neutralization described above may be carried out either before or after mixing the above resin with the blocked polyisocyanate. Acetic acid and formic acid are particularly suited as the neutralizing agent from the viewpoints of a finished appearance, a throwing property and a low temperature-curing property of the coating film.
A blending proportion of the blocked isocyanate to the active hydrogen group-containing cationic resin in the coating composition A of the present invention can be allowed to fall in a range of usually 5 to 40% by weight, preferably 15 to 30% by weight.
The coating composition B of the present invention is a thermosetting type cationically electrodepositable coating composition comprising as a base resin, a cationic resin of a self (internal) cross-linking type having both the blocked isocyanate group represented by Formula (I) described above and an active hydrogen group in a molecule.
The cationic resin described above can be produced, for example, by reacting a resin having an active hydrogen group such as a primary or secondary amino group and a hydroxyl group and a cationic group such as a primary amino group, a secondary amino group, a tertiary amino group and a quaternary ammonium group (when this cationic group contains active hydrogen, it can double as the active hydrogen group) with a polyisocyanate compound which is half-blocked with the amide compound represented by Formula (II) described above to introduce the blocked isocyanate group represented by Formula (I) described above into the above resin. This reaction can be carried out until a free isocyanate group is not substantially detected in the reaction mixture (the presence of an isocyanate group can readily be confirmed by means of infrared spectral analysis).
The amine-added epoxy resin given as the base resin in the coating composition A is suited as the active hydrogen group-containing cationic resin used in the present invention. The aromatic, aliphatic or alicyclic polyisocyanate compounds described in the coating composition A of the present invention are given as the polyisocyanate compound described above. Among them, aromatic diisocyanates such as MDI and crude MDI are suited.
The self (internal) cross-linking type cationic resin thus produced can contain at least 0.1 group, preferably 0.2 to 2.0 groups and more preferably 0.3 to 1.5 group of the blocked isocyanate group represented by Formula (I) per a molecule on the average.
The half-blocked polyisocyanate compound described above can be obtained by reacting MDI or crude MDI with the amide compound represented by Formula (II), if necessary, in an inert organic solvent (for example, ester base solvents, ketone base solvents, aromatic solvents and ether base solvents) which does not substantially react with MDI and the amide compound at a temperature of about 20 to 150xc2x0 C., preferably 30 to 100xc2x0 C. for about 10 minutes to 24 hours, preferably about 20 minutes to 15 hours. In this case, a use proportion of the above amide compound can be allowed to fall usually in a range of 1 to 1.98 mole, preferably 1.05 to 1.95 mole per mole of MDI or crude MDI.
The self (internal) cross-linking type cationic resin produced in such manner as described above is used as the base resin for the coating composition B of the present invention by neutralizing with a neutralizing agent such as aliphatic carboxylic acid in the same manner as described in the coating composition A of the present invention to thereby make it water-soluble or water-dispersible. Acetic acid and formic acid are particularly suited as the neutralizing agent described above from the viewpoints of a finished appearance, a throwing property and a low temperature-curing property of the coating film.
The coating composition A of the present invention and the coating composition B of the present invention (hereinafter called all together the coating composition of the present invention) can further contain, if necessary, a tin compound as a cross-linking acceleration catalyst. The above tin compound includes, for example, organic tin oxides such as dibutyltin oxide and dioctyltin oxide; and aliphatic acid or aromatic carboxylic acid salts of dialkyltin such as dibutyltin dilaurate, dioctyltin dilaurate, dibutyltin diacetate, dioctyltin benzoateoxy, dibutyltin benzoateoxy, dioctyltin dibenzoate and dibutyltin dibenzoate. Among them, dialkyltin aromatic carboxylic acid salts are suitable from a viewpoint of a low temperature curability.
A content of the tin compound in the coating composition of the present invention shall not strictly be restricted and can be changed in a wide range according to performances required to the electrodepositable coating material. Usually, the tin content falls suitably in a range of 0 to 8 parts by weight, preferably 0.05 to 5 parts by weight per 100 parts by weight of the resin solid matters contained in the coating composition of the present invention.
Further, a bismuth compound can be added as a rust preventive to the coating composition of the present invention. The kind of the bismuth compound which can be added shall not specifically be restricted and, to be specific, includes inorganic bismuth compounds such as bismuth hydroxide, basic bismuth carbonate, bismuth nitrate and bismuth silicate. Among them, bismuth hydroxide is particularly preferred.
Further, capable of being used as well are organic acid bismuth salts which are produced by reacting two or more kinds of organic acids with such bismuth compounds as described above and in which at least one of the above organic acids is an aliphatic hydroxycarboxylic acid. The organic acids which can be used for producing the organic acid bismuth salts include, for example, glycolic acid, glyceric acid, lactic acid, dimethylolpropionic acid, dimethylolbutyric acid, dimethylolvaleric acid, tartaric acid, malic acid, hydroxymalonic acid, dihydroxysuccinic acid, trihydroxysuccinic acid, methylmalonic acid, benzoic acid and citric acid.
The inorganic bismuth compounds and the organic acid bismuth salts described above each can be used alone or may be used in combination of two or more kinds thereof.
A content of these bismuth compounds in the coating composition of the present invention shall not strictly be restricted and can be changed over a wide range according to performances required to the coating material. Usually, it can be allowed to fall in a range of 0.01 to 10% by weight, preferably 0.05 to 5% by weight based on the resin solid matters contained in the coating composition of the present invention.
Further, the coating composition of the present invention can be blended, if necessary, with coating material additives such as a color pigment, an extender pigment, a rust preventive pigment, an organic solvent, a pigment dispersant and a coated surface-controlling agent.
The coating composition of the present invention can be coated on a desired base material surface by electrodeposition coating. In general, electrodeposition coating can be carried out on the condition of a loaded voltage of 100 to 400 V in an electrodeposition bath controlled usually to a bath temperature of about 15 to about 35xc2x0 C., which comprises the electrodepositable coating composition of the present invention diluted by adding deionized water so that the solid matter concentration becomes about 5 to about 40% by weight, preferably 15 to about 25% by weight and controlled to a pH falling in a range of 5.5 to 9.0.
A film thickness of an electrodeposited coating film capable of being formed using the coating composition of the present invention shall not specifically be restricted and falls suitably in a range of usually 10 to 40 xcexcm, particularly 15 to 30 xcexcm in terms of a dried coating film. A baking temperature of the coating film falls suitably in a range of usually about 100 to about 200xc2x0 C., preferably about 120 to about 160xc2x0 C.
The electrodeposited coating film capable of being formed using the coating composition of the present invention has a low heating loss and is excellent in a low temperature curability as well as a corrosion resistance. In addition thereto, the coating composition of the present invention has a good storage stability and is useful, for example, as an undercoating material for car bodies, car arts and building members.
The present invention shall be explained below in further details with reference to examples, but the present invention shall not be restricted by them. xe2x80x9c%xe2x80x9d shows xe2x80x9c% by weightxe2x80x9d.